By government regulation there are requirements on the amount of broadcast interference, or noise, which may be generated by electrical devices. Such regulation avoids interference of radio and television broadcasts so that communications, such as those by airports and aircraft, police and fire departments, and ordinary television and radio receivers, are not disrupted.
Thus the limitations imposed by the government (the Federal Communications Commission) are strict. The radiated noise emission limits currently cover the frequency range form 30 MHz to 1000 MHz and are controlled within plus or minus 3 dB of the established limit. The uncertainty of plus or minus 3 dB forces a recommendation that the emission signals from the electric device be at least 6 dB under the limit so that the device can pass the limitation.
Some of the uncertainty is due in part to the inaccuracies in the measurement equipment used to test electrical devices and some are due to the inaccuracies of the open field test site. The radiated noise emission tests are performed under open field conditions where the site of the test is free from obstructions and objects which might cause inaccuracies in the measurement. By description a site must be controlled so that one site behaves and performs as well as another site. Test measurement results at one site should be repeatable at another.
However, the repeatability of site measurement results or a way of verifying that the repeatability of the measurement results at another site is a problem. A more important problem is the comparison of two test site measurements. Should the results at two sites not agree, it is highly desirable to know precisely what the true differences are, say, to within 1 dB.
Thus the problem resolves to one of calibration. Up to now, the calibration of an open field site has taken a lot of expensive equipment; the task of calibration is also labor-intensive. The equipment can cost as much as $100,000; while the time required is usually about 6 hours.
The practical impact of this problem is that many electrical product manufacturers hire outside test laboratory companies to make the measurements on the manufacturer's products. This is especially true with a smaller business which cannot afford to a large investment into this type of test instrumentation. While the manufacturer may start with only one outside laboratory, the manufacturer may end with several test laboratories. Often the first laboratory will have scheduling conflicts and thus the manufacturer is forced to find other test laboratories to meet the pressure of product release deadlines and the like. For the smaller manufacturer which does not have the influence of the larger customers of the laboratory, this is a typical result.
Thus there is an established need for an approach to verify or correlate the results of test site measurements of one laboratory with the results of test site measurements of the second laboratory so that meaningful data can be collected. Without such verification or correlation the determination of whether a product meets the government criteria for electrical noise emissions remains doubtful--a highly unsatisfactory position for the product manufacturer.
The present invention solves or substantially mitigates this problem.